(a) Field of the Invention
The present invention relates to a method for reforming surface of polymer, especially to a method for either providing hydrophilicity or increasing hydrophobicity by reforming a surface of polymer material or polymer membrane.
(b) Description of the Related Art
Polymer membrane is being used widely in various applications such as air purifiers, filter membranes in the water treatment field, electrolysis, separators in batteries, gas exchange membranes, artificial internal organs, beverage purification, enzyme refining, etc.
As separators for batteries, these membranes play a role that isolates an anode from a cathode, and thereby preventing short circuits due to the contact between two electric poles, and at the same time passing an electrolyte or ion. Although separator material itself does not contribute to supplying electrical energy, it does affect the performance and safety of battery by its mechanical characteristics.
There are various required characteristics in a separator of battery that depend on the type of battery and recent varieties of separators have been developed and tried out in secondary, or rechargeable, batteries such as, the lithium battery that requires different characteristics from separators used in conventional batteries.
Basic characteristics required in separators for battery include the ability to isolate the anode and cathode, low electrical resistance for easy passing of electrolyte or ions, superior wettability with electrolyte, adequate mechanical strength for both assembly and use of the battery, the ability to reduce membrane thickness for high density charging, etc. Separator wettability with the electrolyte is especially influential both directly an indirectly on battery production since after the manufacturing of the jelly roll, the process must await the electrolyte to infiltrate into the separator. Therefore, there is much interest in the battery industry with increasing the electrolyte infiltration speed by inducing hydrophilicity into a hydrophobic separator.
Safety features such as shut down characteristics are especially required because highly active organic solvents are used as electrolyte in secondary batteries. Therefore, a new polymeric separator has been developed because cellulose or non-woven fabric used in the conventional batteries could not satisfy the required characteristics of a separator for the above described secondary battery. There are many separators for this purpose which are generally made of polyolefin resin. Lithium ion batteries in particular could be commercialized by using polyolefin resin, which is the only material which has a low reactivity with highly active organic solvents and a low price.
Methods for manufacturing a ventilating film""s precursor film by using polyolefin resin include a wet process where filler or wax and solvent are used to extract low molecular weight material forming pores and a dry process where microscopic pores are formed without using any solvents by stretching at low and at high temperatures. The stretching process related with forming of pores in the dry process frequently employs uni-axial stretching and bi-axial stretching methods. Although there are many processes that can be used theoretically or in laboratories, the only methods for separator manufacturing being used commercially are the above described wet and dry processes.
On the other hand, even though polyolefin based resin is frequently used as separator for lithium ion batteries due to various superior physical properties and economic aspects, this material has a disadvantage in that it has a low wettability with electrolyte because of its inherent hydrophobicity. Therefore, there have been many efforts to counteract the hydrophobicity of polyolefin based membranes.
These include a method for treating membrane surfaces using a surfactant developed by the Hoechst Celanese Company of the U.S.A. and methods for chemically bonding monomer or polymer displaying hydrophilicity to membranes as taught in U.S. Pat. Nos. 3,231,530, 3,853,601, 3,951,815, 4,039,440, 4,340,482, etc.
However, these methods have a lot of problems in that various other unfavorable side chemical reactions cause a molecular weight reduction of polymer membrane that reduces the durability of the manufactured membrane. Furthermore, the process used with these methods are so complicated that it is not advantageous in economic and mass production aspects, as well as producing a poor working environment due to the use of solvents.
Efforts to reform the surface of polymer membranes include a method for imparting hydrophilicity by using a corona, or plasma, etc. Methods for grafting a hydrophilic monomer such as acrylic acid and polymers such as polyethylene oxide to the membrane surface are taught in the U.S. Pat. Nos. 4,346,142, 5,085,775, and 5,294,346. Additionally, a method for plasma treating or sputter etching while at the same time infusing into the membrane surface an oxygen and carbon tetrafluoride (CF4) gas for alkali secondary battery separators that require hydrophilicity together with hydrophobicity in the battery""s discharge characteristics is taught in Japanese Laid-open Patent No. Heisei 8-31399. However, the methods using these plasmas have problems in that control of uniformity is difficult due to plasma""s inherent characteristics of high dependence on external environmental factors, broad energy distribution, etc., as well as the surface may be damaged mechanically due to other side reactions degrading mechanical properties. In other words, the critical physical properties required in a membrane are difficult to achieve using plasmas.
On the other hand, a method for reforming the polymer surface including reducing a contact angle or increasing an adhesion strength of the polymer surface by irradiating it with energized ion particles under vacuum conditions on the polymer surface while at the same time infusing reactive gas into the polymer surface is presented in Korean Laid-open Patent Publication No. 96-37742. This method reports that the contact angle of the polymethylmethacrylate (PMMA) is decreased to 8xc2x0 among the polymers with reformed surfaces, and the contact angle of polycarbonate can be reduced to such a degree that water drops continue to flow, thus making it impossible to measure a contact angle.
However, this method of irradiating ion beams while at the same time infusing a reactive gas is affected by the species of the infused gas selected and the supply of the gas. Furthermore, there are problems in that physical properties of the material, especially the mechanical properties, can deteriorate because the polymer surface can be etched by ancillary ionizations of the coexisting reactive gas in addition to free radical formations on the polymer surface during the ion beam irradiation. Compared to gases used with ion beam, ionized reactive gases are generally known to deepen the degree of etching on a surface during ionization. These phenomena tend to particularly occur more frequently on the surfaces of membranes which can be damaged relatively more easily compared to other materials.
Therefore, it is an object of the present invention considering problems of the conventional technologies to provide a method for reforming the surface of polymer or polymer membrane without deteriorating the mechanical properties of the polymer or the polymer membrane.
It is other object of the present invention to provide a method for reforming polymer or polymer membrane which has a good working environment and is readily applied in mass production techniques owing to easy surface reforming of polymer or polymer membrane without using any solvents.
It is another object of the present invention to provide a method for manufacturing polymer membrane which is used as lithium ion secondary battery separator having good wettability.